Conventional combustion conducted in the presence of a flame, which is usually employed in processes for the combustion of hydrocarbons such as methane, is a procedure that is difficult to control. It occurs in a well-defined range of air/hydrocarbon concentrations and, besides the formation of carbon dioxide and water, it results in the production of pollutants such as carbon monoxide and nitrogen oxides. Catalytic combustion produces few pollutants such as nitrogen oxides (NOX) and carbon monoxide (CO). The introduction of a catalyst permits better control of total oxidation in a wide range of values in respect of the air/hydrocarbon ratio. With a catalyst, the air/hydrocarbon ratio can be outside the limits of inflammability of conventional combustion.
The common causes for catalyst failure include thermal degradation, catalyst poisoning, substrate failure, and plugging, with the two most important modes being thermal degradation and catalyst poisoning. Catalysts are preferably located as close to the exhaust manifold as possible, for fastest activity. However, catalysts located closest to the exhaust manifold are the least durable because they are the most thermally degraded and heavily poisoned. Thermal degradation, e.g., encapsulation and vaporization of the supports(s) and precious metal(s), can be improved with engine controls and more thermally stable materials. Substrate failure and plugging can be reduced with more costly mat material.
Catalyst poisoning, however, cannot be reduced with improved engine controls or more thermally stable materials. Poisons derive from the use of engine oil additives (e.g., zinc, phosphorus, barium, calcium, sodium, and magnesium), deposits of wear metals (e.g., iron, chromium, copper, lead, tin, and silver), and deposits from anti-freeze additives (e.g., silicon, boron, and the like). No matter how well developed the catalyst materials are, these poisons can deposit upon and restrict diffusion to the active metals. As durability requirements increase, emission failures due to poisoning becomes the most encountered failure mode. Further, deactivation is complicated by the fact that not all poisons are equally damaging. Significant amounts of deposits can exist on a catalyst with only marginal catalyst deactivation, e.g., iron and carbon deposits, while small amounts of certain other deposits can result in significant deactivation. For example, contaminants such as zinc, calcium, magnesium, and phosphorus (in forms such as, for example, zinc phosphate or calcium phosphate), and the like, quickly poison or otherwise damage the catalyst.
With increasing durability requirements, there remains a need for improved catalyst protective coatings, and methods for producing the catalyst that reduces poisoning of the precious metals.
Another problem in the formation of a catalyst lies in the overall efficiency of the active phase of the catalyst. Active phases that have a high efficiency for oxidation of hydrocarbons to carbon dioxide and water tend to be less efficient in the downstream reduction of the NOx. Active phases that are less efficient, e.g., those that partially oxidize hydrocarbons to H2, CO, and HCO, tend to be more efficient in the downstream reduction of NOx.